Diatoms are microalgae widely distributed in inland and marine waters and during drowning they are inhaled together with water, passing through the bloodstream to various internal organs of a victim. Our investigation aimed to evaluate the diatoms extracted from the lung tissues of an individual with drowning diagnosis and compare them to those found in the environment, in order to prove the cause of death and the drowning site. A left lung fragment of 17.9 grams removed from the victim was oxidized and prepared for diatoms identification and quantification using inverted microscope. A total of 133 diatoms valves was found, being 85% belong to pennate forms and 15% to centric forms. Pinnularia, a bentonic genus, had greatest number of valves, followed by Fragilaria, Actinocyclus, Cocconeis and Encyonema. These results revealed that the victim drowned at the bottom of the lake, inhaling both water and sediments with diatoms. This is the first investigation of this nature carried out in Brazil.